Heat treatment of light metals



Patented Mar. 21, 1934 UNITED STATES 1,953,003 HEAT TREATMENT or LIGHT METALS Josef Martin Michel, Bitterfeld, Germany,

signor, by mesne assignments, to Magnesi Development Corporation, a corporation of Delaware No Drawing. Application December 4, 1930, Serial No. 499,975. In Germany December 11,

7 Claims. .'(Ol. 148-15) The present invention relates to improvements in means for heating light metals, particularly magnesium and magnesium alloys, in connection with processes for the improvement of mechanical properties of such metals by means of thermal treatment, and is more especially concerned with a new composition to be used for said purpose.

' According to well known processes, metals which are to be subjected to a heat treatment are immersed in fused salts or salt mixtures heated to the requisite temperature for the purpose of rapidly heating such metals and at the same time excluding any possibility of attack by the constituents of the atmosphere upon such heated metals. As a salt of this kind, nitrate of potassium has been frequently proposed, that salt recommending itself for the purpose because of its low melting point and thus rendering heat treatment possible within a comparatively wide temperature range.

However, when employing fused nitrate of potassium for the purpose of heating magnesium and magnesium alloys to temperatures between about 350-400" C.annealing at such temperatures being required for improving certain physical properties of these metals according to a known processit has been ascertained that the work pieces, particularly when in sheet form, are a strongly attacked by the fused salt even when the duration of contact is but comparatively short. It has been observed that the decrease in weight of such sheets-giving a measure of the intensity of the attackranges between about 2.1 and 7.5 grs. per square meter per hour, depending on the duration of contact.

An object of the present invention is to obviate this corrosive action of fused nitrate of potassium compositions upon magnesium and magnesium alloys or work pieces made of such metals. It has been ascertained that the incorporation of chromates and dichromates or fluorides of the alkali metals, even when in comparatively small percentages, such as about 1-2 percent by weight, entirely prevents the corrosive action of the fused nitrate of potassium upon the-metal for all practical purposes. This is probably due to the fact that the presence of such added substances causes the formation of a protective layer upon the surface of the metal.

The following figures show the decided difference produced by the presence, in the fused mixture, of the added protective substances according to the present invention, in comparison with the employment of nitrate of potassium alone.

Example 1 Sheets of a magnesium alloy of equal shape and size were subjected to a contact of 5 to 30 minutes respectively, at a temperature of about 350 C. with fused nitrate of potassium, with or without the added protective substances according to the invention, the decrease in weight corresponding tothe intensity of attack being shown 2. A process of improving the physical proper- I ties of magnesium and magnesium alloys by thermal treatment between about 350-400 0., which comprises subjecting the metal to-contact with a fused composition heated to the desired temperature, said composition chiefly consisting of nitrate of potassium containing from about 1 to 2 percent of a water-soluble dichromate.

3. A composition for the .heat treatment of magnesium and magnesium alloys within the temperature range of about 350-400 C., consisting of nitrate of potassium containing a small percentage of an alkali fluoride.

4. A process of improving the physical properties of magnesium and magnesium alloys by thermal treatment between 350-400" C., which comprises immersing the metal infused potassium nitrate containing a small percentage of a compound selected from the group consisting of chromates, dichromates and fluorides of the alkali metals.

5. A process of improving the physical properties of magnesium and magnesium alloys by thermal treatment between 350-400 C., which comprises immersing the metal in fused potassium nitrate containing a small percentage of a water-soluble chromate.

6. A process of improving the physical properties of magnesium and magnesium'alloys by thermal treatment between 350-400 C., which comprises immersing the metal in fused potassium nitrate containing a small percentage of potassium chromate.

'7. A process of improving the physical properties of magnesium and magnesium alloys by thermal treatment between 350-400 0., which comprises immersing the metal in fused potassium nitrate containing a small percentage of potassium dichromate.

Certificate of Correction Patent No. 1,953,003. March 27, 1934.

JOSEF MARTIN MICHEL I It is hereby certified that error appears in the printed specification of the above numbered patent requiring correction as follows: Page 1, after line 65, insert the following tables: I

Duration of contaot 5 minutes Composition of the Change in weight in grams per and that the said Letters Patent should be read with this correction therein that the same may conform to the record of the case in the Patent Ofiice.

Signed and sealed this 8th day of May, A. D. 1934.

[SEAL] BRYAN M. BATTEY,

Acting Commissz'soner of Patents. 

